It's pretty clear from Kael's survival in Tempest Keep (we'll see about the Sunwell; I'm hoping he betrays Kil'jaeden and fights on your side) and Maiev being the one to finish off Illidan that Blizzard is at least smart enough to not be allowing really major lore characters to just get downed by a raid and looted with no fanfare. And if you don't know that Arthas is the endboss of WotLK by now, I'm guessing you reached this page by mis-clicking a link that you thought would lead to naked pictures.
That said: who would you like to see show up and deliver the deathblow to Arthas?
[The remainder of this post is irrelevant to your answer; feel free to stop reading now.]
There's a good chance that this will just turn into a 'who's your favourite lore character' thread; I'm okay with that too. Having been reading up on the various characters featured in WotLK and the rest of Warcraft's untapped story, I'm kind of hoping for:
Your raid marches on Icecrown Glacier with mighty Rhonin, new leader of Dalaran and the Kirin Tor, at the fore. He has some truly wretched banter with Arthas, who delivers his (admittedly somewhat awesome) 'all shall serve the One True King' line, and then runs him right the @#$% through with Frostmourne, thus allowing Rhonin to serve the legitimate purpose of showing how powerful Arthas really is. The fact that Rhonin would then be totally dead is a beneficial side-effect only.
With your 'leader' dead, waves of Scourge rush the raid in a timed endurance battle, eventually interrupted by a shockwave of light that scythes your part of the battlefield clean. In marches a handful of Silver Hand highlords and Tirion Fordring, carrying the new Ashbringer in one hand and the hammer of Uther the Lightbringer in the other. (Yes, he dual-wields two-handers. Shut up.)
Much cooler banter passes between Fordring and Arthas (someone once wrote a great script that I won't copypasta in here), and the next phase of the battle centres around the duel between them, although the Scourge work on mass numbers, so there are still more reinforcements oncoming.
Ultimately, I think it's pretty clear who needs to actually kill off Arthas.
Jaina Proudmoore has to woman up and bash his face in with Antonidas' staff.
(I would also accept Kael'thas killing Arthas with his bare hands alone.) Either way, the two of them are the closest remaining characters to Arthas, and leaving them out would be like having Vol'jin kill Illidan instead of Maiev.
([EDIT] Zing! I could come up with some explanation of how 'penultimate' actually referred to how Arthas' death is the second-to-last event of the raid encounter, with some kind of denouement to follow between the lore heroes gathered around, but the fact is I wasn't thinking straight and now feel silly.)